Tool and method for digital acquisition of a tibial mechanical axis

ABSTRACT

A tool for digitizing a mechanical axis of a tibia using a computer-assisted surgery system is described. The tool includes upper and lower mounting ends interconnected by an alignment rod extending therebetween. The upper mounting end is releasably fastenable to an upper reference point on a tibial plateau and the lower mounting end includes a self-centering malleoli engaging mechanism having opposed caliper arms displaceable in a common plane relative to each other for clamping engagement with the medial and lateral malleoli of the ankle. At least one trackable member is mounted to the alignment rod of the tool and is in communication with the computer assisted surgery system for providing at least orientation information of the alignment rod. The mechanical axis of the tibia is parallel to the alignment rod and extends between the upper and lower reference points when the tool is mounted on the tibia.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present is a Continuation of U.S. patent application Ser. No.12/872,469 filed Aug. 31, 2010, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The present application relates generally to computer-assisted surgerysystems and, more particularly, to a surgical tool and method used todetermine a tibial mechanical axis using such a CAS system.

BACKGROUND

Computer-assisted surgery (CAS) systems which employ inertial-based ormicro-electro-mechanical sensor (MEMS), trackable members continue to bedeveloped.

One of the principal steps in navigating a bone with inertial sensors isto determine a coordinate system of the bone relative to the sensors,such as to be able to determine the orientation of the bone. For thetibia, the orientation of the bone is determined by its mechanical axis.

When traditional optical CAS navigation systems are used, thedetermination of the tibial mechanical axis can be achieved, forexample, by using two optical bone sensors fixed to the bone at spacedapart locations, each optical sensor having six degrees of freedom (DOF)(i.e. 3 DOF in position and 3 DOF in orientation). When using trackablemembers having inertial sensors in an inertial-based CAS system,however, the inertial sensors do not necessarily provide 6 DOF. Whilethe missing DOF can be calculated if necessary using integratedgyroscope and accelerometer readings, for example, a simpler and moreefficient manner to digitize the mechanical axis of a tibia isnonetheless sought.

Therefore, there remains a need for an improved surgical tool which isused in conjunction with a CAS system in order to digitally acquire themechanical axis of the tibia using readily identifiable anatomicalreference points.

SUMMARY

In accordance with one aspect of the present application, there isprovided a digitizing tool adapted for digitizing a mechanical axis of atibia using a computer-assisted surgery system, the tool comprising: anupper mounting end and a lower mounting end interconnected by analignment rod extending therebetween, the upper mounting end beingreleasably fastenable to an upper reference point on a tibial plateauand the lower mounting end having a self-centering malleoli engagingmechanism thereon, the self-centering malleoli engaging mechanismincluding opposed caliper arms each having a malleolus clamp, thecaliper arms being displaceable in a common plane relative to each otherfor clamping engagement with the medial and lateral malleoli, such thata midpoint between the caliper arms corresponds to a lower referencepoint defined midway between the medial and lateral malleoli; and atleast one trackable member mounted to the alignment rod of the tool, thetrackable member producing at least orientation information and beingadapted for communication with the computer-assisted surgery system;wherein the alignment rod is aligned parallel with the mechanical axisof the tibia which extends between the upper reference point and thelower reference point when the tool is mounted on the tibia.

In accordance with a further aspect of the present application there isalso provided a method for determining a mechanical axis of a tibiausing an inertial-based computer assisted surgery system and a tibialdigitizer having an upper mounting end, a lower mounting end and analignment rod extending therebetween, the tibial digitizer including atleast one inertial sensor in communication with the computer assistedsurgery system, the method comprising: determining an upper referencepoint on a tibial plateau of the tibia, the upper reference point beingan entry point of the mechanical axis; fastening the upper mounting endof the tibial digitizer to the tibial plateau at the upper referencepoint; fastening the lower mounting end of the tibial digitizer tomedial and lateral malleoli of the ankle; determining a lower referencepoint located at a midpoint between the medial and lateral malleoli byidentifying a corresponding midpoint on the lower mounting end of thetibial digitizer; adjusting an orientation of the alignment rod suchthat the alignment rod is aligned with an anatomical landmark on thetibia; and using the computer assisted surgery system to determine themechanical axis of the tibia extending between the upper and lowerreference points by providing at least orientation data of the tibialdigitizer to the computer assisted surgery system using the inertialsensor.

There is further provided, in accordance with another aspect of thepresent application, a digitizing tool adapted for acquiring amechanical axis of a tibia using a computer assisted surgery system, thedigitizing tool comprising: spaced apart upper and lower mounting endseach having at least one mounting point respectively adapted to besecured to a tibial plateau and malleoli of the tibia, the mountingpoint of the upper mounting end being fasteneable to the tibial plateauat a mechanical axis entry point defining an upper reference point; analignment rod extending between and interconnecting the upper and lowermounting ends, the alignment rod defining a longitudinal axis and beingdisposed a common distance from the mounting points on the upper andlower mounting ends, at least one trackable member mounted to thealignment rod, the trackable member producing at least orientation-baseddata for at least two degrees of freedom in orientation of the trackablemember and thus of the alignment rod; the lower mounting end including aself-centering malleoli engagement mechanism having a base portionpivotally connected to the alignment rod and opposed caliper armsslideably mounted on the base portion for displacement relative to eachother in a plane transverse to the longitudinal axis of the alignmentrod, wherein the caliper arms, when displaced towards each other, areadapted to abut the most medial and lateral points on the malleoli toclamp the self-centering malleoli engagement mechanism in place thereto;and wherein a midpoint between the caliper arms of the self-centeringmalleoli engagement mechanism corresponds to a lower reference pointlocated at a midpoint between the most medial and lateral points on themalleoli, and the mechanical axis extends between the lower referencepoint and the upper reference point at said common distance from thealignment rod which is aligned parallel thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an x-ray of a side view of a tibia showing the tibialmechanical axis;

FIG. 2 is a partial side view of an ankle region showing the malleoliand a midpoint therebetween used as a first reference point fordetermining the mechanical axis of the tibia;

FIG. 3 is a top plan view of the tibial plateau, showing the entry pointthereon used as a second reference point for determining the mechanicalaxis of the tibia;

FIG. 4 is a perspective view of a tibial digitizer tool of the presentapplication used to acquire the mechanical axis of the tibia;

FIG. 5 is a perspective view of a tibial reference which is installed onthe tibial plateau in alignment with the mechanical axis of the tibiausing the tibial digitizer tool of FIG. 4;

FIG. 6 is a perspective view of a self-centering malleoli engagingmechanism on a lower portion of the tibial digitizer tool of FIG. 4;

FIG. 7 is a side view of the tibial digitizer and tibial referencemounted to a tibia in order to digitally acquire its mechanical axisusing a CAS system in communication therewith;

FIG. 8 is a perspective view of the tibial digitizer and tibialreference mounted to the tibia;

FIG. 9 is a perspective view of the tibial reference tool being mountedto the tibial plateau at a mechanical axis entry point thereon;

FIG. 10 is a side perspective view of a lower portion of the tibialreference tool being axially displaced towards the ankle;

FIG. 11 is a enlarged perspective view of the self-centering malleoliengaging mechanism on the lower portion of the tibial digitizer toolbeing adjusted to engage the malleoli of the ankle;

FIG. 12 is an enlarged perspective view of an upper portion of thetibial reference tool being adjusted in a medial-lateral direction suchas to visually align the opening in the tibial digitizer tool with thetibial tuberosity which is used as an anatomical landmark;

FIG. 13 is a side perspective view of the tibial digitizer tool beingremoved from the tibial reference still fixed to the tibial plateau;

FIG. 14 is an enlarged perspective view of a MEMS sensor of the CASsystem being fastened to the tibial reference;

FIG. 15 is a flow chart illustrating a method for determining themechanical axis of the tibia in accordance with an embodiment of thepresent disclosure;

FIG. 16 is a perspective view of an alternate embodiment of a tibialdigitizer tool used to acquire the mechanical axis of a tibia;

FIG. 17 is an enlarged front perspective view of a self-centeringmalleoli engaging mechanism on a lower portion of the tibial digitizertool of FIG. 16;

FIG. 18 is an enlarged rear perspective view of the self-centeringmalleoli engaging mechanism on a lower portion of the tibial digitizertool of FIG. 16; and

FIG. 19 is an enlarged perspective view of top portion of the tibialdigitizer tool of FIG. 16.

DETAILED DESCRIPTION

The term “CAS” is used herein to refer to computer-assisted surgery.

The term “MEMS” is used herein to refer to micro-electro-mechanicalsensors, for example, but not limited to, accelerometers, gyroscopes andother inertial sensors.

The present surgical tool and method will be generally described hereinwith respect to use of the device in conjunction with an inertial-basedCAS system 100 employing trackable members having inertial-basedsensors, such as the MEMS-based system and method for tracking areference frame disclosed in U.S. Patent Application No. 61/309,585filed on Mar. 2, 2010, and the MEMS-based system and method forplanning/guiding alterations to a bone disclosed in U.S. patentapplication Ser. No. 12/410,884 filed Mar. 25, 2009, the entire contentsof both of which are incorporated herein by reference. However, it is tobe understood that the tool and method described herein may also be usedwith other CAS systems.

The surgical tool 10 (best seen in FIG. 4) is a “tibial digitizer”,which may, in a particular embodiment, be provided for use with aninertial-based CAS system in order to digitally acquire the mechanicalaxis of the tibia. Thus, as will be described, the tibial digitizer 10includes trackable members thereon which, in at least the presentlydescribed embodiment, include inertial sensors for communication withthe inertial-based CAS system. These inertial sensors are referred to asMEMS sensors or MEMS trackable members in the embodiment describedbelow, however it is to be understood that the term “MEMS” or “MEMSsensor” as used herein may include any combination of inertial-basedtracking circuitry, for example including MEMS, gyroscopes,accelerometers, compasses, electronic tilt sensors, etc., all of whichare able to detect orientation changes. However, although particularlydeveloped for use with inertial based sensors and an inertial-based CASsystem, it is also to be understood that the present tibial digitizermay similarly be used with other CAS systems, and thus may includetrackable members thereon which are not exclusively inertial-based. Aswill be described in further detail below, the tibial digitizer 10 isused to digitally acquire the mechanical axis of the tibia, in a mannerwhich is quick, accurate and easily repeatable, by using readilyidentifiable anatomical references on the tibia to position the tibialdigitizer in place, as will be first described with reference to FIGS.1-3.

As seen in FIG. 1, the mechanical axis Z of the tibia T may in fact bedefined by two reference points located from known landmarks on thebone. As seen in FIG. 2, the first, or lower, of these two referencepoints is the midpoint P₁ between the most medial point 5 on the medialmalleolus and the most lateral point 7 of the lateral malleolus (on thefibula F) which make up the ankle. The second, or upper, of these tworeference points is the mechanical axis entry point P₂ on the tibialplateau 9. The generally accepted mechanical axis entry point on thetibial plateau may be used. However, in one particular embodiment, themechanical axis entry point P₂ on the tibial plateau 9 may be defined asbeing at the intersection of two axes on the tibial plateau, the firstaxis Y being centered medial-laterally and the second axis X beinglocated one-third anterior and two-thirds posterior. Thus, themechanical axis Z of the tibia T is defined between the two referencepoints P₁ and P₂, which can be located and acquired by the CAS systemfor the tibia T using the identified anatomical landmarks which arelocated by the tibial digitizer tool 10.

Referring now to FIGS. 4 to 8, the tibial digitizer 10 will be describedin further detail. As seen in FIG. 4, the tibial digitizer 10 generallyincludes an upper mounting end 12 and a lower mounting end 14,interconnected by an alignment rod 18. The upper mounting end, or upperportion, 12 is removably fastened to the tibial plateau using a tibialreference 20 (see FIG. 5). The lower mounting end, or lower portion, 14engages the ankle region, and more specifically the malleoli, using aself-centering malleoli engaging mechanism 16, as will be described infurther detail. In one embodiment, the alignment rod 18 whichinterconnects the upper and lower portions of the tibial digitizer isadjustable in length along its longitudinal axis 19, such as to permitthe upper and lower portions 12, 14 to be axially displaced relativelyto each other along this longitudinal axis 19, while being nonethelesscapable of being fixed in place once a desired length of the tibialdigitizer 10 is reached. For example, the alignment rod 18 may be atelescoping rod, and/or may, as depicted in FIG. 4, be slideablyreceived within a mating tube 17 of the upper portion 12, in order topermit longitudinal adjustment along the axis 19. This longitudinaladjustment permits the overall size of the tibial digitizer tool 10 tobe adjusted as required in order to fit a large range of tibia lengths.A locking mechanism 15 is provided on the upper portion 12 in order tofix the sliding rod 18 of the lower portion 14 and the tube 17 of theupper portion 12 in place with respect to each other, therebymaintaining the desired overall axial length of the tibial digitizer 10such that it accommodates the tibia being operated on. The alignment rod18 is, in the embodiment described herein, pivotally mounted to both theupper and lower mounting ends 12,14, such as to permit an orientation ofthe alignment rod 18 to be adjustable once the upper and lower mountingends are fastened in place to the tibia.

The tibial reference 20 shown in FIG. 5 is fastened to the tibialplateau 9 (as shown in FIG. 9 for example). More specifically, thetibial reference 20 is fastened to the tibial plateau 9 at the upperreference point P₂ (see FIG. 3) corresponding to the mechanical axisentry point using a suitable number of pins or other bone fasteners (forexample two or three fasteners 21 may be used). At least one of thesefasteners 21 is fixed to tibial plateau 9 at the mechanical axis entrypoint P₂ such that the mechanical axis entry point of the tibia is at aknown position with regard to the tibial reference 20, and therefore tothe tibial digitizer 10 once engaged to the tibial reference 20. Thetibial reference 20 may additionally serve, subsequent to the digitalacquisition of the tibial axis as described herein, as a mounting pointand platform for deployment of a cutting guide or other cuttingreference block used during a knee replacement surgery to resect aportion of the tibia in preparation for the installation of a tibialknee prosthesis.

In this regard, the innermost end 13 of the upper portion 12 of thetibial reference 10 includes a releasable engagement mechanism 11thereon, which is used to releasably fasten the tibial digitizer 10 tothe tibial reference 20 in order to removably fasten the tibialdigitizer 10 to the tibia. Thus, with the tibial reference 20 pinned tothe mechanical axis entry point P₂ on the tibial plateau 9, theuppermost end of the tibial digitizer 10 is fastened in place to thetibia at the upper tibial axis entry point P₂.

The releasable engagement mechanism 11 between the tibial digitizer 10and the tibial reference 20 may include at least two rotationaladjustments, namely one in the flexion-extension direction and one inthe varus-valgus direction. These two rotational adjustments permit theentire tibial digitizer 10 to be pivoted in these two degrees offreedom, as will be described in further detail below, while the upperportion 12 of the tibial digitizer 10 nonetheless remains fastened inplace to the tibial plateau 9. As will be seen, the adjustment in theflexion-extension and varus-valgus planes permits the tibial digitizer10 to be adjusted as required when the malleoli engagement mechanism 16of the lower portion 14 of the tool is engaged in place on the ankle.

Referring now to FIG. 6, the self-centering malleoli engaging mechanism16 is disposed at the lower end of the rod 18 to form the lower portion14 of the tibial digitizer 10. The self-centering mechanism 16 comprisesa clamp-like caliper 22 having a base portion 25, pivotally mounted to alower end of the rod 18 by a pivotal connection 30, and opposed caliperarms 24 which are slideably mounted on the base portion 25 fordisplacement relative to each other in a plane that is substantiallytransverse to the longitudinal axis 19 of the rod 18, i.e. the axis 19intersects the plane within which the caliper arms move at any anglethat may include, but is not limited to, 90 degrees. In at least onepossible embodiment, the caliper arms 24 are displaced in asubstantially medial-lateral direction 26 in order to bring the malleoliclamps 28, disposed on the inner-most remote ends of each caliper arm24, into abutted engagement with the most medial point on the medialmalleolus and the most lateral point on the lateral malleolis.Accordingly when so engaged, the self-centering mechanism 16 is clampedin place on the malleoli and thereby able to define a mid-point betweenthe malleoli while clamped in place thereon, given that a midpointbetween the caliper arms 24 also corresponds to a midpoint between themedial and lateral malleoli. The lower reference point P₁ (see FIG. 2)can therefore be identified by the CAS system by identifying themidpoint of the self-centering mechanism 16 on the lower portion 14 ofthe tibial digitizer 10. In one possible embodiment, at least one of thecaliper arms 24 of the self-centering malleoli engaging mechanism 16includes an inertial-based trackable member 54 thereon, such as toproduce orientation data pertaining to at least one degree of freedom inorientation of the caliper arms 24 in their plane of sliding adjustment.This accordingly permits the CAS system in communication with thetrackable member 54 to determine the orientation of the caliper arms 24,and thus the entire self-centering malleoli engaging mechanism 16.

In one embodiment, the arms 24 of the caliper 22 may operate in aratchet-like manner, in that they can be displaced inwardly (i.e.towards each other) until the clamps 28 engage the malleoli, howeveroutward displacement of the caliper arms 24 is prevented unless alocking feature released by the operator. Namely, as seen in FIG. 11, aratchet mechanism 27 may be provided within the base portion 25 of theself-centering mechanism 16, and the inner ends of each of the caliperarms 24 are engaged by the ratchet mechanism 27 such that inwardmovement of the caliper arms is permitted but outward movement (i.e.away from the malleoli) is prevented unless a release lever 29 isactuated. Thus, once located and fastened in place on the malleoli, theself-centering mechanism 16 remains fixed in place until such time asthe release lever 29 is actuated, thereby permitting the caliper arms 24to be moved apart and the self-centering mechanism 16 thus released fromengagement with the ankle. Alternately, the centering malleoli engagingmechanism may include another type of mechanism permitting the caliperarms to be displaced in an inward direction towards each other withoutrestriction such as to clamp onto the malleoli but restricting movementof the caliper arms in an opposed outward direction unless the lockingfeature of the mechanism is released. For example, the mechanism may bespring-loaded or otherwise biased in the closed position.

The lower end of the rod 18 is pivotally linked with the self-centeringmechanism 16 via a pivot connection 30, which may include a sliding pindisplaceable within a mating slot 32 in the base portion 25 of theself-centering mechanism 16. Alternately, the pivot connection 30 may belocated in a fixed medial-lateral direction on the base portion 25,while nonetheless still permitting pivotal movement in a varus-valgusplane between the rod 18 and the base 25 of the self-centering mechanism16.

Once the upper end 12 of the tibial digitizer 10 has been fastened tothe tibial reference 20 and the self-centering mechanism 16 of thetibial digitizer's lower portion 14 has been clamped in place on themalleoli, the tibial digitizer 10 is thus fastened to the tibia at bothupper and lower ends thereof. As best seen in FIG. 12, the rod 18 of thedigitizer may then be rotated, for example in a medial-lateral directionand/or varus-valgus plane, such as to visually align a visual referenceguide in the form of a visual guide 36, which in this embodiment is anopening defined through the rod 18 and/or the tube 17 of the tibialdigitizer's upper portion 12 within which the rod is received, with aselected anatomical landmark on the tibia. In a particular embodiment,the anatomical landmark used is the tibial tuberosity 3, as shown inFIG. 12. In alternate embodiments, other landmarks may also be used, forexample the anterior crest of the tibia, however this may requirereconfiguration of the alignment rod and/or the location, shape and/orconfiguration of the visual markers 36. Accordingly, the visual guide 36is disposed on the alignment rod 18 in a position corresponding to aproximal-distal location of an anatomical landmark on the tibia used toposition and/or align the alignment rod relative thereto. Although inthe present embodiment the visual guide 36 is an opening extendingthrough the rod 18 and/or the tube 17, it is to be understood thatanother suitable visually identifiable guide may be used, for example areference marker located on an outer surface of the rod and/or tube 17.

When so adjusted, the orientation of the rod 18 is fixed in place, suchthat it remains in fixed position relative to both the lower end 14clamped in place to the malleoli and the upper end 12 fixed in place tothe tibial plateau via the tibial reference 20, and therefore relativeto the tibia as a whole.

At least the orientation of rod 18 of the tibial digitizer 10 may thenbe determined by the CAS system 100 (FIG. 8), which is in communicationwith at least one inertial-based trackable member 50 mounted on the rod18, such as to thereby digitally acquire the mechanical axis Z of thetibia which, as noted below, is now disposed parallel to the tracked rod18 of the tibial digitizer 10.

Once adjusted in position as shown in FIGS. 7 and 8, the rod 18 of thetibial digitizer 10 is thereby located at a known distance D away fromboth the lower reference point P₁, located at midpoint of theself-centering mechanism 16, and the upper reference point P₂, locatedat the mechanical axis entry point on the tibial plateau at which thetibial reference 20 is fastened. As such the rod 18 of the tibialdigitizer 10 is aligned with, and parallel to, the mechanical axis Z ofthe tibia. Once in this position and orientation, the MEMS trackablemember 50 on the tibial digitizer 10, which is positioned on the rod 18and/or the tube 17 receiving the rod 18, the digitizer 10 can thereforebe used by the CAS system 100 in communication with the MEMS sensor 50to determine the location and orientation in space of the mechanicalaxis Z of the tibia T when the tibial digitizer 10 is mounted to thetibia as described herein. The tibial digitizer 10 accordingly permitsthe CAS system to digitally acquire and subsequently track themechanical axis Z of the tibia.

Referring now to FIGS. 9 to 14, the method of installing the tibialdigitizer 10 as set out in FIG. 15 will now be described in furtherdetail.

As seen in FIG. 9, once the tibial reference 20 is fastened in place tothe selected mechanical axis entry point P₂ using one or more fasteners21 (in one exemplary embodiment two or three are used), the upperportion 12 of the tibial digitizer 10 may then be engaged to the tibialreference 20 by the attachment mechanism 11. Once the upper portion 12of the tibial digitizer is fastened in place at the upper end of thetibia, as seen in FIG. 10, the lower portion 14 of the tibial digitizer,including the self-centering malleoli engaging mechanism 16, is axiallydisplaced in direction 40 towards the ankle by sliding the rod 18 out ofthe tube 17 of the tibial digitizer's upper portion 12. As seen in FIG.11, once the self-centering mechanism 16 on the lower portion of thetibial digitizer is proximate the ankle, the rod 18 may be fixed inposition within the tube 17 using the locking mechanism 15 (not seen inFIG. 11) and the self-centering mechanism 16 of the lower portion 14 ofthe tibial digitizer may be adjusted as described above in order for themalleoli clamps 28 on the caliper arms 24 to be aligned with, and movedtowards each other until they engage, the medial and lateral malleoli.The self-centering mechanism 16 therefore permits the caliper arms 24 tobe displaced inwardly such that the clamps 28 engage the malleoli, whilethe central portion 25 of the self-centering mechanism 16 remainspivotally connected to the rod 18 via the pivot connection 30. Byclosing the caliper arms 24 and engaging the malleoli, theself-centering mechanism 16 is thereby able to identify the midpointbetween the two clamps 28, which accordingly corresponds to the lowerreference point P₁ defined at the midpoint between the most medial point5 on the medial malleolus and the most lateral point 7 on the lateralmalleolus. The lower reference point P₁ of the mechanical axis is thusdefined and identified in space by the CAS system, such that themechanical axis Z of the tibia may be determined.

As seen in FIG. 12, once the upper and lower ends of the tibialdigitizer 10 are fastened to the bone, medial-lateral displacementand/or varus-valgus rotational adjustment of the rod 18 (not seen inFIG. 12), and tubular portion 17 with which the rod is mated, ispermitted by the pivoting connections at either end of the tibialdigitizer. This medial-lateral displacement and/or varus-valgusrotational adjustment is used to visually align and center the openings36, defined in the tubular portion 17 of the tibial digitizer's upperportion 12, with the tibial tuberosity 3 on the tibia in order to ensurethat the rod 18 and tube 17 of the tibial digitizer 10 are aligned withand parallel to the mechanical axis of the tibia, as described abovewith reference to FIG. 7.

Once in position, the MEMS trackable member(s) 50 on the tibialdigitizer 10 provide at least two degrees of freedom information to theCAS system in communication with the tracked tibial digitizer 10, suchthat the CAS system may then digitally acquire the position andorientation of the mechanical axis of the tibia. Once this is achieved,the tibial digitizer 10 may be disengaged from the tibial reference 20,which remains fastened to the tibial plateau, as shown in FIG. 13. Oncethe tibial digitizer has been removed, a further MEMS sensor 52 may thenbe fastened to the tibial reference 20, as shown in FIG. 14. The MEMSbone sensor 52 remains fixed relative to the tibia and relative to theacquired mechanical axis thereof, in order to permit the CAS system tofurther track the tibia during surgery.

As set out in FIG. 15, and with reference to FIGS. 4-14 and thepreceding description of the tibial digitizer 10, the method 110 ofdetermining a mechanical axis of a tibia using an inertial-basedcomputer assisted surgery system and the present tibial digitizer willnow be summarized. As noted above, the tibial digitizer 10 includes anupper mounting end 12, a lower mounting end 14 and an alignment rod 18extending therebetween. The tibial digitizer 10 includes at least oneinertial sensor 50 in communication with the inertial-based computerassisted surgery system 100. A first step 112 of the method 110 includesdetermining an upper reference point P₂ on a tibial plateau 9 of thetibia T, the upper reference point P₂ being an entry point of themechanical axis Z. Step 114 of the method includes fastening the uppermounting end 12 of the tibial digitizer 10 to the tibial plateau 9 atthe upper reference point P₂, as shown in FIG. 9. Step 116 of the methodincludes fastening the lower mounting end 14 of the tibial digitizer 10to medial and lateral malleoli of the ankle, as shown in FIG. 11. Step118 of the method includes determining a lower reference point P₁located at a midpoint between the medial and lateral malleoli byidentifying a corresponding midpoint on the lower mounting end 14 of thetibial digitizer 10. Step 120 of the method includes adjusting theorientation of the alignment rod 18 such that it is aligned with ananatomical landmark on the tibia, for example the tibial tuberosity 3,as shown in FIG. 12. Step 122 of the method includes using the CASsystem to determine the mechanical axis Z of the tibia, which extendsbetween the upper and lower reference points, by providing at leastorientation data of the tibial digitizer 10 to the CAS system using theinertial sensor 50.

The step 122 may also include determining at least orientation of thetibial digitizer 10 using the computer assisted surgery system 100 suchas to digitally acquire the mechanical axis Z of the tibia based on thedetermined orientation of the alignment rod 18 and the known distance D(see FIG. 7) between the trackable member 50 on the tibial digitizer 10and the mechanical axis.

The step 120 may also include orienting the alignment rod 18 to beparallel with the mechanical axis Z of the tibia extending between thedetermined upper and lower reference points P₂, P₁.

The step 112 may also include locating the entry point of the mechanicalaxis on the tibial plateau 9 by identifying an intersection point, whichcorresponds to upper reference point P₂, between a first axis Y centeredmedial-laterally on the tibial plateau 9 and a second axis X locatedone-third anterior and two-thirds posterior, as seen in FIG. 3. Thesefirst and second axis X,Y lie in a common plane transverse to themechanical axis Z.

The step 116 may include using the self-centering clamp mechanism 16 onthe lower mounting end 14, and inwardly displacing the opposed caliperarms 24 of the self-centering clamp mechanism 16 towards each otheruntil they abut and are clamped to the medial and lateral malleoli 5,7of the ankle.

The step 120 may include aligning the anatomical landmark on the tibiawith a visual guide defined on the alignment rod 18, and moreparticularly displacing the alignment rod 18 until the tibial tuberosity3, which is the anatomical landmark used in one embodiment, is centeredwithin opening 36 of the visual guide. This adjustment and displacementof the alignment bar 18 may include at least one of a translation in amedial-lateral direction and a rotation in a varus-valgus plane, or anycombination thereof.

The step 110 as described herein is, in one particular embodiment,performed entirely on a bone model or cadaver.

The present application features CAS trackable members, such as the MEMStrackable member 50 for example, which are inertial-based sensors andwhich therefore include inertia-based tracking circuitry. The trackingcircuitry within these trackable members may featuremicro-electromechanical sensors (MEMS), gyroscopes, accelerometers orother types of inertial sensors (electrolytic tilt sensors, compasses)to detect orientation changes, for instance of the tibial digitizer 10.Therefore, while MEMS sensors 50, 52 are described herein as oneparticular embodiment of the present disclosure, it is understood thatany suitable inertial-based sensor may be used. These inertial sensorsmay include, for example and without being limited to: tri-axialgyroscopic sensors in an orthogonal or semi-orthogonal configuration aswell as tri-axial accelerometer sensors in an orthogonal orsemi-orthogonal configuration.

The CAS system 100 in communication with the inertial sensors of thetrackable members 50, 52, 54 which constitute the tracking membersobtains planar (i.e. orientation) information and optionally positioninformation directly from the inertial MEMS sensors of these trackablemembers, rather than having to compute this information as would berequired when using conventional optical tracking members. In otherwords, the inertial sensors provide at least two degrees of freedom inorientation, and optionally up to three degrees of freedom in position.

Referring now to FIGS. 16 to 19, a tibial digitizer 210 in accordancewith an alternate embodiment operates much as per the tibial digitizer10 described above and depicted in FIGS. 4 to 8.

The tibial digitizer 210 generally includes an upper mounting end 212and a lower mounting end 214, interconnected by an alignment rod 218.The upper mounting end, or upper portion, 212 is removably fastened tothe tibial plateau using a tibial reference 220. The lower mounting end,or lower portion, 214 engages the ankle region, and more specificallythe malleoli, using a self-centering malleoli engaging mechanism 216. Inthis embodiment, the upper mounting end 212 is displaceable on thealignment rod 218 along its longitudinal axis, such as to permit theupper and lower portions 12, 14 to be axially displaced relatively toeach other along this longitudinal axis, while being nonetheless capableof being fixed in place once a desired relative position of the upperand lower mounting ends are reached. This longitudinal adjustmentpermits the overall size of the tibial digitizer tool 210 to be adjustedas required in order to fit a large range of tibia lengths. A lockingmechanism 215 (see FIG. 19) is provided on the upper portion 212 inorder to fasten the upper portion 212 in place on the rod 218, therebymaintaining the desired overall axial length of the tibial digitizer 210such that it accommodates the tibia being operated on. The alignment rod218 is, in this embodiment, pivotally mounted to both the upper andlower mounting ends 212,214, such as to permit an orientation of thealignment rod 218 to be adjustable once the upper and lower mountingends are fastened in place to the tibia.

The tibial reference 220 is adapted to be fastened in place to the tibiaat the mechanical axis entry point on the tibial plateau. The tibialreference 220 is releasably fastened to the upper portion 212 of thetibial digitizer 210, and permits at least two rotational adjustments,namely one in the flexion-extension direction and one in thevarus-valgus direction. These two rotational adjustments permit theentire tibial digitizer to be pivoted in these two degrees of freedomwhile the upper portion 212 of the tibial digitizer 210 nonethelessremains fastened in place to the tibial plateau via the tibialreference.

Referring now to FIGS. 16 to 17, the self-centering malleoli engagingmechanism 216 is similar to the self-centering malleoli engagingmechanism 16 described above. The self-centering malleoli engagingmechanism 216 comprises a clamp-like caliper 222 having a base portion225, pivotally mounted to a lower end of the rod 218 by a pivotalconnection 230, a central caliper body portion 231 and a pair of opposedcaliper arms 224 on either side of the central caliper body portion 231.The caliper arms 224 are slideably mounted on the central caliper body231 for displacement relative to each other within a common plane thatmay be substantially transverse to the longitudinal axis of the rod 218.The caliper arms 224 are may therefore be displaced in a substantiallymedial-lateral direction 226 in order to bring the malleoli clamps 228,disposed on the inner-most remote ends of each caliper arm 224, intoabutted engagement with the most medial point on the medial malleolusand the most lateral point on the lateral malleolis. In an alternateconfiguration, however, the caliper arms 224 may be pivotably broughttowards each other rather than remaining relatively parallel to eachother when displaced within their common plane, as in the case in thepresent embodiment. Regardless, when the two caliper arms 224 arebrought towards each other until they abut the two malleoli, theself-centering mechanism 216 is clamped in place on the malleoli andthereby able to define a mid-point between the malleoli while clamped inplace thereon, given that a midpoint between the caliper arms 224 alsocorresponds to a midpoint between the medial and lateral malleoli. Thecentral caliper body portion 231 is further adjustable relative to thebase portion 225 via a pivoting adjustment, such that the entire calipersub-assembly may be pivoted in the angular direction 221 within theabove-mentioned common plane that is transverse to the longitudinal axisof the alignment rod 218. The pivoting adjustment may include otherconfigurations, but in the present embodiment includes a sliding pindisposed on the base portion 225 which is slidably displaceable within amating slot 232 in the base portion 25 of the self-centering mechanism16. The caliper arms 224 of the self-centering melleoli engagingmechanism 216 may include one or more of the inertial-based trackablemembers described above.

The self-centering mechanism 216 includes a mechanism which permits thearms 224 of the caliper 222 to be displaced in an inward directiontowards each other without restriction while at least restrictingmovement of the caliper arms in an opposed outward direction unless alocking feature of the mechanism is released. This mechanism whichpermits only one way direction, or which at least provides lessresistance to movement in one direction, may include for example aratchet mechanism and a spring-loaded mechanism. In the case of aratchet-mechanism, the caliper arms 224 can be displaced inwardly (i.e.towards each other) until the clamps 228 engage the malleoli, howeveroutward displacement of the caliper arms 224 is prevented unless alocking feature is released by the operator. Alternately, in the case ofa spring-loaded mechanism, the caliper arms may be inwardly biased, suchas by a spring or other equivalent member, such that their tend toreturn to their inwardly directed clamped position unless they arereleased via a deactivated locking feature or are otherwise separated bythe user.

The presently described tibial digitizer tool 10 is therefore used todigitize the mechanical axis of the tibia, thereby creating anorientation reference such as to enable subsequent tracking of theacquired mechanical axis of the tibia by the CAS system, using forexample the trackable member 52 fastened to the tibia post-digitizationof the mechanical axis. The inertial or MEMS trackable members of thedigitizer accordingly provide two or three DOF tracking circuitry or canalternately be calibrated to perform orientation tracking, such that theCAS system in communication with these sensors is able to digitallyacquire the mechanical axis of the tibia and thus subsequently track thetibia during surgery.

The presently described MEMS-based trackable members may include both agyroscope sensor and an accelerometer sensor, and thus may providereadings to the CAS system from both types of sensors. The gyroscopesensor and the accelerometer sensor within the trackable members mayeach provide at least orientation data along three degrees of freedom.

The embodiments of the invention described above are intended to beexemplary only. Those skilled in the art will therefore appreciate thatthe forgoing description is illustrative only, and that variousalternatives and modifications can be devised without departing from thescope of the present invention, which is intended to be limited solelyby the scope of the appended claims. Accordingly, the presentdescription is intended to embrace all such alternatives, modificationsand variances which fall within the scope of the appended claims.

1. A digitizing tool adapted for digitizing a mechanical axis of a tibiausing a computer-assisted surgery system, the tool comprising: an uppermounting end and a lower mounting end interconnected by an alignment rodextending therebetween, the upper mounting end being releasablyfastenable to an upper reference point on a tibial plateau and the lowermounting end having a self-centering malleoli engaging mechanismthereon, the self-centering malleoli engaging mechanism includingopposed caliper arms each having a malleolus clamp, the caliper armsbeing displaceable in a common plane relative to each other for clampingengagement with the medial and lateral malleoli, such that a midpointbetween the caliper arms corresponds to a lower reference point definedmidway between the medial and lateral malleoli; and at least onetrackable member mounted to the alignment rod of the tool, the trackablemember producing at least orientation information and being adapted forcommunication with the computer-assisted surgery system; wherein thealignment rod is aligned parallel with the mechanical axis of the tibiawhich extends between the upper reference point and the lower referencepoint when the tool is mounted on the tibia.
 2. The tool as defined inclaim 1, wherein the trackable member includes an inertial sensor. 3.The tool as defined in claim 2, wherein the inertial sensor produces atleast orientation-based data for at least two degrees of freedom inorientation of the sensor and thus of the alignment rod on which thesensor is mounted.
 4. The tool as defined in claim 2, wherein theinertial sensor includes one or more micro-electro-mechanical sensors,accelerometers, gyroscopes, compasses, and electronic tilt sensors. 5.The tool as defined in claim 1, wherein the self-centering malleoliengaging mechanism includes at least a second trackable member thereon,the second trackable member produces orientation-based data of theself-centering malleoli engaging mechanism.
 6. The tool as defined inclaim 1, wherein the self-centering malleoli engaging mechanism includesa mechanism permitting the caliper arms to be displaced in an inwarddirection towards each other without restriction such as to clamp ontothe malleoli but at least restricting movement of the caliper arms in anopposed outward direction unless a locking feature of the mechanism isreleased.
 7. The tool as defined in claim 6, wherein the mechanismincludes one of a ratchet mechanism and a spring-loaded mechanism. 8.The tool as defined in claim 1, wherein the alignment rod defines alongitudinal axis extending therethrough, the alignment rod beingadjustable in length relative to the longitudinal axis such as to vary adistance between the upper and lower mounting ends.
 9. The tool asdefined in claim 8, wherein the alignment rod telescopes such as toadjust the length thereof.
 10. The tool as defined in claim 1, whereinthe alignment rod is pivotally mounted to both the upper and lowermounting ends, such as to permit an orientation of the alignment rod tobe adjustable in at least a varus-valgus plane once the upper and lowermounting ends are fastened in place to the tibia.
 11. The tool asdefined in claim 1, wherein the alignment rod includes a visual guidethereon, the visual guide being disposed on the alignment rod in aproximal-distal position corresponding to a location of an anatomicallandmark on the tibia used to align the alignment rod.
 12. The tool asdefined in claim 11, wherein the visual guide includes an openingextending through the alignment rod within which the anatomical landmarkis visually centered to align the alignment rod with the mechanical axisof the tibia.
 13. A method for determining a mechanical axis of a tibiausing an inertial-based computer assisted surgery system and a tibialdigitizer having an upper mounting end, a lower mounting end and analignment rod extending therebetween, the tibial digitizer including atleast one inertial sensor in communication with the computer assistedsurgery system, the method comprising: determining an upper referencepoint on a tibial plateau of the tibia, the upper reference point beingan entry point of the mechanical axis; fastening the upper mounting endof the tibial digitizer to the tibial plateau at the upper referencepoint; fastening the lower mounting end of the tibial digitizer tomedial and lateral malleoli of the ankle; determining a lower referencepoint located at a midpoint between the medial and lateral malleoli byidentifying a corresponding midpoint on the lower mounting end of thetibial digitizer; adjusting an orientation of the alignment rod suchthat the alignment rod is aligned with an anatomical landmark on thetibia; and using the computer assisted surgery system to determine themechanical axis of the tibia extending between the upper and lowerreference points by providing at least orientation data of the tibialdigitizer to the computer assisted surgery system using the inertialsensor.
 14. The method as defined in claim 13, wherein determining themechanical axis further comprises determining at least orientation ofthe tibial digitizer using the computer assisted surgery system such asto digitally acquire the mechanical axis of the tibia based on thedetermined orientation of the alignment rod of the tibial digitizer anda known distance between the inertial sensor on the tibial digitizer andthe mechanical axis.
 15. The method as defined in claim 13, whereinadjusting the orientation of the alignment rod further comprisesorienting the alignment rod to be parallel with the mechanical axis ofthe tibia extending between the determined upper and lower referencepoints.
 16. The method as defined in claim 13, wherein locating theupper reference point further comprises locating a mechanical axis entrypoint on the tibial plateau.
 17. The method as defined in claim 16,wherein locating the mechanical axis entry point includes identifying anintersection point between a first axis centered medial-laterally on thetibial plateau and a second axis located one-third anterior andtwo-thirds posterior, the first and second axis lying in a common planetransverse to the mechanical axis.
 18. The method as defined in claim13, wherein the lower mounting end of the tibial digitizer includes aself-centering clamp mechanism, the step of fastening the lower mountingend further comprising inwardly displacing opposed caliper arms of theself-centering clamp mechanism towards each other until the caliper armsabut the medial and lateral malleoli of the ankle.
 19. The method asdefined in claim 13, wherein adjusting an orientation of the alignmentrod further comprises aligning the anatomical landmark on the tibia witha visual guide defined on the alignment rod.
 20. The method as definedin claim 19, wherein the anatomical landmark is a tibial tuberosity andthe visual guide is an opening defined in the alignment rod, the step ofadjusting further comprising displacing the alignment rod until thetibial tuberosity is centered within the opening of the visual guide.21. The method as defined in claim 20, wherein displacing the alignmentrod includes at least one of a translation in a medial-lateral directionand a rotation in a varus-valgus plane.
 22. The method as defined inclaim 13, wherein the method is performed on a bone model or cadaver.23. A digitizing tool adapted for acquiring a mechanical axis of a tibiausing a computer assisted surgery system, the digitizing toolcomprising: spaced apart upper and lower mounting ends each having atleast one mounting point respectively adapted to be secured to a tibialplateau and malleoli of the tibia, the mounting point of the uppermounting end being fasteneable to the tibial plateau at a mechanicalaxis entry point defining an upper reference point; an alignment rodextending between and interconnecting the upper and lower mounting ends,the alignment rod defining a longitudinal axis and being disposed acommon distance from the mounting points on the upper and lower mountingends, at least one trackable member mounted to the alignment rod, thetrackable member producing at least orientation-based data for at leasttwo degrees of freedom in orientation of the trackable member and thusof the alignment rod; the lower mounting end including a self-centeringmalleoli engagement mechanism having a base portion pivotally connectedto the alignment rod and opposed caliper arms slideably mounted on thebase portion for displacement relative to each other in a planetransverse to the longitudinal axis of the alignment rod, wherein thecaliper arms, when displaced towards each other, are adapted to abut themost medial and lateral points on the malleoli to clamp theself-centering malleoli engagement mechanism in place thereto; andwherein a midpoint between the caliper arms of the self-centeringmalleoli engagement mechanism corresponds to a lower reference pointlocated at a midpoint between the most medial and lateral points on themalleoli, and the mechanical axis extends between the lower referencepoint and the upper reference point at said common distance from thealignment rod which is aligned parallel thereto.
 24. The digitizing toolas defined in claim 23, wherein the trackable member includes aninertial sensor.
 25. The digitizing tool as defined in claim 24, whereinthe inertial sensor includes one or more micro-electro-mechanicalsensors, accelerometers, gyroscopes, compasses, and electronic tiltsensors.
 26. The digitizing tool as defined in claim 23, wherein theself-centering malleoli engaging mechanism includes at least a secondtrackable member thereon having a second inertial sensor, the secondinertial sensor producing at least orientation-based data of theself-centering malleoli engaging mechanism for one or more degrees offreedom in orientation of the second inertial sensor independently ofthe first inertial sensor.
 27. The digitizing tool as defined in claim26, wherein the second inertial sensor is mounted on one of the caliperarms of the self-centering malleoli engagement mechanism.
 28. Thedigitizing tool as defined in claim 23, wherein the self-centeringmalleoli engaging mechanism includes a mechanism which permits thecaliper arms to be displaced in an inward direction towards each otherwithout restriction for clamping onto the malleoli but at leastrestricting movement of the caliper arms in an opposed outward directionunless a locking feature of the mechanism is released.
 29. Thedigitizing tool as defined in claim 28, wherein the mechanism includesone of a ratchet mechanism and a spring-loaded mechanism.
 30. Thedigitizing tool as defined in claim 23, wherein the alignment rod islongitudinally adjustable such as to vary a length thereof.
 31. Thedigitizing tool as defined in claim 30, wherein the alignment rodtelescopes to provide longitudinal adjustment between the upper andlower ends along the longitudinal axis.
 32. The digitizing tool asdefined in claim 23, wherein at least an orientation of the alignmentrod is adjustable in at least a varus-valgus plane once the upper andlower ends are fastened in place to the tibia.
 33. The digitizing toolas defined in claim 23, wherein the alignment rod is pivotally mountedto both the upper and lower mounting ends, permitting orientation of thealignment rod to be adjustable once the upper and lower mounting endsare fastened in place to the tibia.
 34. The tool as defined in claim 5,wherein the second trackable member is mounted on one of the caliperarms of the self-centering malleoli engagement mechanism.